Multicarrier transmission schemes, such as Orthogonal Frequency Division Multiplexing (OFDM), may use a number of orthogonal subcarriers to carry data. The superposition of the orthogonal subcarriers may lead to large peaks in the magnitude of the signal, which may be measured as Peak to Average Power Ratio (PAPR), and commonly expressed in decibels (dB). In case of Single-Carrier Frequency Division Multiple Access (SC-FDMA) modulation, superposition of very long symbol pulses may also lead to increased PAPR.
To transmit a modulated radio signal, the base-band (BB) complex digital signal may be converted into an analog signal with digital-to-analog converter (DAC), then frequency-shifted into a radio frequency band centered at a radio carrier, and amplified by a power amplifier (PA). If the BB digital signal has a large PAPR, without having a large back-off (defined as the ratio of PA's saturation power/average PA power) value in the PA, the signal peaks may be distorted, and the signal may have spectral leaks into adjacent channels. These spectral leaks, also known as out-of-band transmissions, may be undesirable and may be prohibited by transmission control authorities of various countries and regions, such as the Federal Communications Commission (FCC) of the United States of America. However, having a large PA back-off value may degrade PA efficiency and output power.
The PAPR of the BB signal may be reduced by a digital “clipping” (which may include limiting magnitude of the signal samples while preserving their phases) process, followed by a digital low pass filter (LPF) suppressing the spectral leaks resulting from the clipping. However, the LPF filtering may have the adverse effect of increasing the peak magnitude, and consequently, the PAPR, of the signal, thereby reversing at least part of the PAPR reduction done by the clipping. As such, the clipping and LPF-filtering may need to be repeated several times in order to achieve the desired PAPR reduction. Such an iterative process may be expensive, or otherwise impractical for various reasons.